1. Field of the Invention
The invention relates to a process for preparing trichloromonosilane from particulate silicon and gaseous hydrogen chloride.
2. Background Art
The demand for ultrapure silicon for the semiconductor industry and the photovoltaic industry has risen sharply in the last few years. From the various production routes, deposition from trichlorosilane with hydrogen has been found to be a commercially attractive variant. Thus, 90% of the silicon used in the semiconductor field is prepared with the aid of purified trichlorosilane. The preparation of trichlorosilane from metallurgical silicon is carried out in fluidized bed reactors. This is described, for example, in U.S. Pat. No. 4,092,446, in which hydrogen chloride flows through a silicon bed consisting of silicon particles. The hydrogen chloride reacts with the silicon particles to give tetrachlorosilane and trichloromonosilane and hydrogen. However, the patent is silent with respect to either the particle size distribution needed in the reactor or the optimal particle size distribution necessary to achieve a very high trichloromonosilane content in the product gas. Both unconverted hydrogen chloride and high by-production of tetrachlorosilane increase the process costs considerably.
ULLMANNS ENCYKLOPÄDIE DER TECHNISCHEN CHEMIE [ULLMANN'S ENCYCLOPEDIA OF TECHNICAL CHEMISTRY], published in 2002, describes the preparation process in such fluidized bed reactors at a reactor temperature of 650° C. Hydrogen chloride reacts with the silicon particles at this temperature preferentially to give trichloromonosilane and hydrogen.
DE A1 3938897 shows that the tetrachlorosilane:trichloromonosilane ratio is dependent upon the temperature maintained in the reactor. It is also demonstrates that, with an increase in the homogeneity of the silicon particles used, the proportion of hydrogen chloride gas in the product gas is only 3% and a product lower in SiCl4 is obtained. According to DE A1 3938897, it is advantageous to use a silicon powder which has been obtained by gas atomization of molten Si and has a particle size distribution between 1 and 1000 μm, or preferably between 50 and 800 μm, to prepare trichloromonosilane in a fluidized bed reactor.
In contrast to a fluidized bed process, DE C2 3239590 describes a moving bed process for preparing trichlorosilane and tetrachlorosilane. In this case, the silicon is present in lump form.
UK patent 945,618 discloses the use of silicon particle size fractions of from 30 to 500 μm, preferably from 50 to 200 μm, for preparing trichloromonosilane in a fluidized bed reactor. This commercially available material may be purified further before use.
US 2002/0151737 A1, corresponding to EP A1 1 249 453 discloses the initial introduction of Si dust into a liquid and only then into a reactor (Muiller-Rochow or trichloromonosilane synthesis). This process is costly and inconvenient, and the silicon powder to be used is brought to the necessary fineness by a grinding step.